The present invention relates to a method of lateral alignment of the cross-direction profile control of a web as required by a papermaking process, in a paper machine, in which a certain cross-direction profile of the dried web, particularly the basis weight profile thereof, to be aligned is gauged or measured, and the profile measurement signal thus obtained is passed to a control system of the paper machine which provides a control signal suited to control the adjustment means of the cross-direction profile control provision. Also, the web is provided with at least one marker line, whose lateral shift or shifts is/are detected at the measurement point of the cross-direction profile of the dried web, or in the vicinity thereof. The detection of the lateral shift(s) is used to generate a measurement signal of the shift(s) which signal is employed to control the lateral alignment of the web profile adjustment.
Furthermore, the present invention relates to a control apparatus for the adjustment and alignment of the cross-direction profile of a web manufactured in a paper machine. The control apparatus comprises a measurement beam or equivalent element adapted to be positioned at the dry end of the paper machine, most advantageously in the vicinity of the reel-up station, a control system to which a measurement signal of the cross-direction profile of the web from a measurement sensor or sensors of the measurement beam is passed, and means which facilitate the cross-direction adjustment of the stock flow profile at the wet end of the paper machine (i.e., in the vicinity of the headbox), most advantageously utilizing a feedback signal formed from the measurement signal obtained from the control system mentioned in the foregoing reference to the paper machine headbox. Also, the apparatus may include an applicator apparatus of a marker line or lines to be made onto the web, sensor means adapted in conjunction with or to the vicinity of a measurement beam or equivalent element, whereby the sensor means is capable of measuring the lateral shift(s) of the marker line(s). The apparatus also has an arrangement suited to control the lateral alignment of the stock flow profile adjustment provision on the basis of the measurement signal indicating the lateral shift(s).
Conventionally, a stock mixture is admitted via the headbox slice of paper or board machines in the form of a suspension jet onto a forming wire in a forming section or into a nip formed between two forming wires in the forming section. The cross-direction profile of the headbox slice determines the cross-direction profile of the discharged stock flow. The slice profile is adjustable and this slice profile control is also capable of compensating for those defects of the stock flow that occur in the headbox or stages preceding it.
Control systems for a paper or board machine are known in the art and are used for the adjustment of a certain cross-direction quality profile of the web being manufactured, particularly its basis weight profile, whereby such a control system comprises a plurality of actuators and a corresponding number of actuator control means. The actuators are arranged to function over the entire width of the web whose profile is to be adjusted. This type of prior art control system incorporates a process control computer or similar logic controller and a feedback loop including the measurement arrangement for the controlled cross-direction profile of the web.
As to the state-of-the-art for above-mentioned control systems of a paper machine, reference is made to Finnish Laid-Open Publication No. 85,731 (corresponding to the assignee's U.S. Pat. No. 5,381,341, the specification of which is hereby incorporated by reference herein, and European Patent Publication No. 0 401 188) filed earlier by the assignee. These documents disclose such a paper machine control system in which the individual actuators are provided with intelligent actuator controllers, and the information transfer in the control hierarchy of the system between a higher level control unit and the controllers of the individual actuators is implemented using a common bus. The control scheme of this control system is based on the distributed intelligence of the actuator controllers, which is parametrized only by the set values issued by the higher-level control system. Each actuator controller is seen by the higher-level system as an individual unit to which the set value is sent via the serial bus in digital format, after which the actuator controller takes care of the mechanical actuation in a self-contained manner based on its stored measurement/control algorithm.
The requirements set on the evenness of cross-direction profiles of both coated and uncoated paper are today tighter than ever primarily due to the elevated quality standards of printing processes and printed material.
However, profile control implemented by means of the lip adjustment of the headbox slice is hampered by certain shortcomings, i.e., that variations in the gap width between the slice lips cause cross-direction flow components in the jet flow of the discharging stock that in turn affect the evenness of the cross-direction profile of the fiber formation in the web. Accordingly, it is desirable to run the headbox with slice profile of maximally constant gap width. Due to these and other reasons, the tendency has recently been to develop and install so-called dilution headboxes in which the basis weight control of the web is principally implemented by controlling the cross-direction consistency profile of the stock flow discharged from the headbox. For practical embodiments of dilution headboxes, reference is made to, e.g., Finnish Patent No. 92228 and Finnish Patent Application No. 942780 filed by the assignee herein (which corresponds to U.S. Pat. No. 5,545,293).
Such consistency profile adjustment is implemented by feeding diluting water to those points of the web formation where the basis weight is higher than average via, e.g., manifold channels of the turbulence generator of the headbox. A problem associated with the use of dilution headboxes may arise therefrom that the web undergoes cross-direction "floating" during its formation and drying process so that the consistency profile adjustment performed based on the basis weight profile measured close to the reel-up end of the paper machine will be laterally misaligned, whereby a lateral shift of the profile control occurs that is extremely detrimental to the end result of the profile adjustment.
When the term dilution headbox is mentioned in the foregoing and later in the text, this term must be understood to generally refer to such headboxes in which cross-direction consistency profile adjustment of the stock flow is used. Such adjustment may also be implemented so that in addition to or instead of the dilution water, controlling stock flows may alternatively be used having a consistency different from the average consistency of the stock in the headbox. Also stock with a consistency higher than the average may be applied via the auxiliary feeds of the cross-direction profile adjustment provision. Lateral shift of the web is caused by the cross-direction shrinkage of the web occurring during the drying cycle of the web that is nonuniform over the width of the web. Such lateral shift is also partially caused by the lateral shifts of the web-supporting fabrics of the paper machine as well as the lateral velocity components of stock flow in the headbox slice channel and the discharged jet.
The dominating cause of the above-mentioned lateral shift is traceable to web shrinkage in the dryer, or more generally, any drying shrinkage in the formation of the web. Maximally the web shrinkage is in the order of about 20 cm to about 40 cm. Moreover, the higher web speeds of modern paper machines elevate the tendency of developing larger web speed differentials along the web path, which further results in length variations of the wires and, hence, the tendency of causing a contracted section in the web. Consequently, the cross-direction shrinkage of the web may be caused by both the drying process and the web speed differentials between the different wire groups along the web path.
The accuracy and stability of the lateral alignment in the control of web basis weight and other similar profiles becomes problematic particularly in conjunction with paper grade changes at the paper machine. It is conventional to operate such paper machines in which during each day the number of grade changes may mount up to several tens. By means of prior art control systems, the above-described lateral alignment of profile adjustment provision has been a difficult and time-consuming operation, whereby also the accuracy of such alignment leaves room for improvement. Such shortcomings may lead to lower availability of paper machines and even paper quality problems.
Conventionally, the lateral alignment of the cross-direction profile has been implemented using a method in which the adjustment screw of a certain headbox slice section is operated to cause a distinct change in the slice gap width at the adjustment screw and the effect of the change is measured with the help of the measurement beam of the basis weight profile at the dry end of the paper machine close to the reel-up station. This alignment method is hampered by its inaccuracy, since the change in the basis weight profile caused by means of the adjustment screw is extremely faint and flat.
Also known in the art for the above-mentioned lateral alignment is such a manual method in which a marker agent is injected into the stock jet discharged from the headbox and the lateral shift of the mark thus generated is detected, e.g., visually.
With regard to the state-of-the-art related to the present invention, reference is made to German Patent Publication No. DE 40 08 282 A1 (assigned to J. M. Voith GmbH). This publication discloses a method and apparatus employed for the lateral alignment of a cross-direction property profile of a paper web similar to that defined in the introductory part of the present patent application. This publication describes an injection header of marker lines suited to be placed at the dryer section of a paper machine, whereby the injection header is used to inject over the entire width of the paper web a series of mutually parallel marker lines, which are employed to determine the cross-direction shrinkage of the paper web. Additionally, the German publication mentions casually that such a shift measurement of the marker lines can be used for the control of the headbox slice lip. However, the principal content of the German publication relates to the control of web moisture profile modifying equipment of the dryer section along the paper web path such as steam boxes or infra-red radiant heaters.
One particular shortcoming of the method and apparatus disclosed in above-mentioned German publication is its incapability of determining the effect of cross-direction flow components occurring inside the paper machine headbox on the alignment of the cross-direction basis weight profile of the paper web. This disadvantage has been found particularly problematic in the dilution headbox, or consistency profile controlled headbox, which was mentioned above and will be described later in greater detail, because provided that the afore-mentioned shortcoming could be removed, this type of headbox can offer more accurate and defined control of basis weight than is conventional in the art.